1. Field of the Invention
This invention relates generally to analog-to-digital converters and to disk drives for computers and, more specifically, to an integrated circuit for analog-to-digital conversion of servo signals in a disk drive.
2. Description of the Related Art
An analog-to-digital converter (ADC) is a circuit that receives a voltage or current input and, in response, produces a digital output that corresponds to the input. An ADC typically includes a digital-to-analog converter (DAC), which is a circuit that receives a digital input and, in response, produces a voltage or current that corresponds to the input.
Several types of ADCs are known in the art. The simplest type of ADC is known as a single-slope or ramping ADC. A ramping ADC includes a counter, a DAC and a comparator. The DAC receives the output of the counter. The comparator receives the input voltage and the output of the DAC. The counter begins counting upwards from a value of zero and stops when the comparator indicates that the voltage corresponding to the count equals the input voltage. The count at which that condition occurs is the digital output that corresponds to the input voltage. Another type of ADC, known as a tracking ADC, is similar to a ramping ADC, but the counter counts either upwards or downwards, thereby tracking changes in the input voltage.
Another type of ADC is known as a successive-approximation ADC. A successive-approximation ADC includes a DAC, a comparator, and a successive approximation register (SAR). The comparator receives the input voltage and the output of the DAC. The DAC receives the output of the SAR. On the first cycle of the conversion, the SAR begins by producing a digital output that is one-half the full-scale value. On the second cycle the SAR produces an output that is halfway between its previous output and zero if the comparator indicates that the input voltage is less than the output of the DAC, and produces an output that is halfway between its previous output and the full-scale value if the comparator indicates that the input voltage is greater than the output of the DAC. On successive cycles, the SAR continues to produce outputs halfway between two previous outputs or between a previous output and zero or between a previous output and the full-scale value. A successive approximation ADC thus performs a binary search until it arrives at a digital output that corresponds most closely to the analog input. Successive approximation ADCs therefore have faster average conversion times than ramping ADCs.
Several types of DACs are known in the art. The simplest type of DAC is known as a weighted-resistor DAC because it includes a network of resistors connected to a summing node. The bits of the input word are connected to corresponding switches, such as transistors. Each switch connects one of the resistors into the network if the corresponding bit is high ("1") and disconnects the resistor from the network if the corresponding bit is low ("0"). The resistor has a value that is weighted according to the position of the bit. The summing node sums the currents contributed by the resistors that are switched into the network, thereby producing a current that corresponds to the digital input. An op-amp is typically included to convert this current into a voltage output. A modified type of weighted-resistor DAC, known as a R-2R ladder DAC, has a resistor network that minimizes the range of resistor values. Similar types of DACs are known that use a capacitor network rather than a resistor network. A related type of DAC, known as a binary-weighted current sink DAC, includes weighted current sources rather than weighted resistors. Each bit controls a group of transistors, such as bipolar transistors or metal-oxide semiconductor field-effect transistors (MOSFETs), corresponding in number to the weight of the bit. The number of transistors in the group controlled by the n.sup.th bit is 2.sup.n. The gates of the MOSFETs in each group are coupled to each other and to the controlling bit. The sources and drains of the MOSFETs in each group are also coupled to each other. An N-bit binary-weighted current sink DAC thus requires 2.sup.N -1 transistors.
A DAC is typically implemented in an integrated circuit or chip. The precision of the resistors and the offset voltage of the transistors of a DAC affect its accuracy. Current sink DACs are preferred because they minimize the number of resistors and thus the cumulative error. Current sink DACs can be implemented using common chip fabrication processes such as the bipolar metal-oxide semiconductor (BiMOS), complementary metal-oxide semiconductor (CMOS), and bipolar complementary metal-oxide semiconductor (BiCMOS) process. CMOS and BiCMOS chips are advantageous because they are more power-efficient than many other processes. Nevertheless, it is difficult and thus relatively uneconomical to produce precision resisters using the CMOS and BiCMOS processes.
The servo system of a computer disk drive includes a servo demodulator and an ADC. The servo system receives servo burst signals from certain transducer heads and uses those signals to determine the radial position of the heads on the disk. The disk drive microprocessor can determine the position of the heads in response to the integral of the servo burst over a certain time period. The ADC, which is typically eight to 10 bits wide, converts the integral to a digital value and provides it to the microprocessor. In certain servo systems, the servo demodulator estimates this integral by full-wave rectifying the servo burst, detecting the peaks, and estimating the integral based on an assumption that the waveform is known. In other servo systems, the servo demodulator determines the integral by full-wave rectifying the servo burst and providing the rectified signal to an analog integrator circuit. The latter type of servo demodulator is more accurate and is known as an area integrating servo demodulator (AISD). The AISD is conventionally implemented with analog components in a BiCMOS integrated circuit or chip. Commercially available BiCMOS servo chips that form the basis for an AISD include, for example, the Analog Devices AD7775.
It would be desirable to provide an ADC that is economical, accurate and power-efficient. It would further be desirable to provide an ADC that interfaces with the AISD in an economical manner and suitable for integration onto the same microcircuit chip. These problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.